The present invention relates generally to an eddy current braking system for a rotational body such as a brake drum, and more particularly, to such a system using permanent magnets or electromagnets to enhance braking efficiency.
An eddy current braking system generates a braking force in response to the mutual action of magnetic fields applied by magnets to a brake drum via ferromagnetic plates and eddy currents generated by relative rotation between the magnets and the brake drum. If a material having a large permeability such as iron or steel and a material having large electric conductivity such as copper are combined in the brake drum, levels of eddy current and braking force increase.
In prior attempts to improve braking efficiency, a copper plate, a copper coating, electric conductive rods or a copper ring have been mounted on the outer surface of each ferromagnetic plate (pole piece), each permanent magnet, or on the inner surface of the brake drum. For example, as shown in FIG. 9, with a tubular body 46 made of copper is secured to the inner surface of a brake drum 13 made of steel, the braking force is increased to some extent. However, because of the poor heat transfer existing at the central portion of the inner surface of the brake drum 13, the adjacent central portion of the tubular body 46 experiences significant increases of temperature resulting in thermal deterioration and an accompanying problem in reliability. Specifically, since the copper constituting the tubular body 46 has a higher thermal expansion coefficient than that of the steel constituting the brake drum 13, high temperature at the central portion of the inner surface of the brake drum 13 causes peeling (separation) or cracking at the joint between the tubular body 46 and the brake drum 13.
In an attempt to overcome the foregoing problem there has been made an arrangement shown in FIG. 10 and disclosed in Japanese Utility Model Application Laid-Open No. Hei-4-111284 Publication. That system employs channel-shaped copper rings 47 fitted over both side surfaces of a brake drum 13. In addition, a plurality of axially directed electrically conductive rods 48 are disposed between the copper rings 47 to dissipate heat from the central portion of the brake drum 13. It has been found, however, that the temperature of the electrically conductive rods 48 are not significantly reduced nor is braking performance significantly enhanced. Particularly, in the case where the copper rings 47 are fitted over opposite end surfaces of the brake drum 13, a clearance existing therebetween increases electrical resistance and braking performance is not improved. Further, more undesirable high frequency noise is generated during braking operations. Even if the direction of the electric conductive rods 48 are inclined relative to the rotational axis of the brake drum 13 as disclosed in Japanese Utility Model Application Laid-Open No. Hei 3-874186 Publication, the high frequency noise is not completely eliminated.